The invention relates to electrical insulating sheet material and to insulated electrical windings incorporating such sheet material. More particularly, the invention relates to an insulating sheet material that can be thermally cured under high pressure without causing the sheet material or any of its component parts to either creep or be extruded from between members on which the material is bonded.
It is common practice in the manufacture of insulated electrical windings, such as those used in electrical motors or in power transformers, to insulate the respective turns of the windings from one another by placing insulating sheet material between the winding turns. Such sheet material insulation is normally only required on high voltage windings or windings having relatively large turns which inherently develop relatively high voltages between the adjacent turns of the winding. For a number of years sheets of asbestos material have been used to provide this dielectric function in making large windings for rotating dynamoelectric machines or for high voltage electric transformers. While asbestos sheet material possesses a number of excellent characteristics for this application, it is relatively difficult and expensive to handle in many manufacturing environments. Moreover, a significant disadvantage involved in the use of asbestos sheet material to insulate the winding turns of such high voltage apparatus is that the insulating sheet must be cut in sections to completely cover facing sides of adjacent turns of the windings. Inevitably, undesirably large amounts of airborne asbestos particles are formed by such cutting operations so it is necessary to perform the operations in carefully controlled environments that protect the respiratory systems of workers from damage that might be inflicted by the airborne asbestos particles.
It is also a typical practice in the manufacture of prior art high voltage electrical windings to use resinous adhesive materials to form bonds between winding turns and sheets of dielectric material positioned between the respective turns of the windings. A wide variety of materials have been used with different degrees of success to perform this bonding function on given types of high voltage electrical windings. However, prior to the present invention, all known combinations of resin bonding adhesives and dielectric sheet material have exhibited certain common disadvantages. In addition to the problem of airborne asbestos particles just mentioned, generally speaking, known combinations of dielectric sheet material and resinous bonding adhesives typically present the following common types of manufacturing problems. They are difficult to handle and accurately position between the turns of electrical windings during the initial manufacturing phases of a winding operation. Further, these known prior art insulating materials tend to creep or extrude from between the turns of the windings when the windings are subjected to high pressure compression during normal bonding operations that are employed to cure the bonding resins and to uniformly distribute the resins over the adjacent surfaces of the winding turns. Also, some combinations of prior art insulating sheet materials do not exhibit as long a shelf life or as desirable a bonding strength as is normally preferred in the manufacture of high voltage electrical windings such as those used on dynamoelectric machines or high voltage transformers. Finally, many of the known prior art insulating sheet materials become thermally unstable when subjected to more than a narrow range of temperatures or when subjected to given temperatures for an extended period of time.
Of course, it is desirable to improve the thermal stability and thermal aging characteristics of insulating sheet material used in the manufacture of high voltage electrical windings so that their overall operating characteristics will be enhanced and extended. It would also be desirable to provide an insulating sheet material that can be easily and efficiently applied to form an excellent dielectric barrier between adjacent turns of high voltage windings with a minimum of application effort and required labor content. Specifically, it would be desirable to provide such a material that can be quickly and easily applied to completely insulate adjacent turns of windings from one another and at the same time to form an insulating barrier between the winding turns which will not require additional treatment after it is cured in order to place the insulated windings in condition for assembly into a rotating dynamoelectric machine or a power transformer. For certain types of windings it has been found to be desirable to fold sheets of insulating material around the edges of the winding turns, thus, it is necessary to provide insulating sheet material that can be folded in 180 degree bends without fracturing, tearing or losing its insulating properties at the folds.